In an effort to integrate an entire transceiver system used in a communication system into a single integrated circuit (IC), a complementary metal-oxide semiconductor (CMOS) power amplifier is being widely used. The CMOS power amplifier generally employs a plurality of CMOS transistors in a cascode configuration. The CMOS transistors of the CMOS cascode power often share a high drain voltage swing and, therefore, problems resulting from junction breakdown of the CMOS transistors can be ameliorated.
Furthermore, in an effort to improve efficiency of a radio frequency (RF) power amplifier, a supply-voltage modulated power amplifier, such as an envelope tracking power amplifier or an envelope elimination and restoration power amplifier, is being used.
FIG. 1 is a simplified schematic diagram of a known supply-voltage modulated power amplifier 100. The supply-voltage modulated power amplifier 100 of FIG. 1 includes a power amplification unit 110 for amplifying an RF input signal, an envelope detection unit 120 for detecting an envelope of the RF input signal, and an envelope tracking power circuit 130 for supplying the RF power amplification unit 110 with a modulated, supply voltage based on a change in the envelope over time. As such, the known supply-voltage modulated power amplifier 100 is not operated by a fixed supply voltage, but is operated by a modulated supply voltage based on an envelope of the RF input signal, that is, a variable supply voltage, as a drive power.
However, there are several problems in applying the known supply-voltage modulated power amplifier to the above-described CMOS cascode power amplifier. Notably, when the CMOS cascode power amplifier is operated by a variable supply voltage, it is difficult to maintain a gate voltage of each of a plurality of cascode transistors at an optimum gate bias point thereof. Specifically, in the CMOS cascode power amplifier, each of the cascode transistors has an optimum gate bias point that enables the CMOS cascode power amplifier to operate at an optimum efficiency. Further, the optimum gate bias point varies in proportion to the supply voltage. However, bias networks, such as a resistive divider and a current mirror, which are generally used to bias gate voltages of a known CMOS cascode power amplifier, cannot render the gate voltage of each of the cascode transistors to the optimum gate bias point when the supply voltage varies. The reason for this is that a voltage coupling from a drain of each of the cascode transistors to a gate thereof, which results from the variable supply voltage, changes a gate bias point of each of the cascode transistors rendered by said bias networks. It is difficult to predict the amount of the voltage coupling because it significantly varies with a size of the cascode transistor and a layout of the CMOS cascode power amplifier.
Moreover, the efficiency of the CMOS cascode power amplifier may significantly deteriorate at a low supply voltage. Specifically, the efficiency of the supply-voltage modulated power amplifier tends to deteriorate under low-supply voltage conditions because of a “knee” voltage and a non-linear gate-to-drain capacitance. Meanwhile, the knee voltage in the CMOS cascode power amplifier is higher than that in other devices, increasing in proportion to the number of cascode transistors. Furthermore, when the supply voltage decreases, the gate-drain capacitance increases, and consequently, an optimum load impedance may vary. In case of the CMOS cascode power amplifier, a variation amount of the optimum load impedance tends to increase in proportion to the number of cascode transistors, and therefore, the efficiency of the CMOS cascode power amplifier may further decrease.
As a result, in order to improve the efficiency of the supply-voltage modulated CMOS cascode power amplifier, it is necessary to maintain a gate bias of each of the cascode transistors at an optimum gate bias point thereof and to suppress increases of a knee voltage and a non-linear gate-to-drain capacitance at a low supply voltage even when using a variable supply voltage as a drive power.
What is needed, therefore, is a power amplifier that overcomes at least the shortcomings of known power amplifiers described above.